Among producers of acrylic pressure sensitive adhesives (PSAs) there is a trend toward reducing the proportion of solvent in the production process. This relates in particular to the coating process, since here in general the polymers are coated from a solution with a concentration of 20 or 30% onto the corresponding carrier material and subsequently the solvent is distilled off again in drying tunnels. As a result of the heat introduced, the drying step may additionally be utilized for the thermal crosslinking of the PSA.
If it is then desired to reduce the solvent fraction or to eliminate it completely, polyacrylate PSAs can be coated from the melt. This is done at relatively high temperatures, since otherwise the flow viscosity would be too high and the adhesive would exhibit an extreme resilience during the coating operation. One example of a functioning commercial system is represented by the UV acResins™ from BASF AG. Here, a low flow viscosity at temperatures of less than 140° C. has been achieved by lowering the average molecular weight to below 300 000 g/mol. Accordingly, these materials are easy to coat from the melt. As a result of the lowering, however, there is also a deterioration in the technical adhesive properties, especially the cohesion, of these PSAs. In principle, the cohesion can be raised by UV or EB crosslinking. Nevertheless, the UV acResins™ do not achieve the level of cohesion attained by high molecular mass acrylic PSAs which have been applied conventionally from solution and crosslinked thermally.
A key problem is the network arc length, since acrylic hotmelt PSAs generally have a relatively low molecular weight, possess a relatively low fraction of interloops, and thus need to be crosslinked to a greater extent. Although the greater crosslinking does increase the level of cohesion, the distance between the individual crosslinks becomes smaller and smaller. Consequently, the network is significantly tighter and the PSA then possesses only a low level of viscoelastic properties.
Accordingly, there is a need for a polymer which is easy to coat from the melt and is subsequently crosslinked on the carrier material in film form in a specific way, so that, preferably, a linear polymer with only very few crosslinking sites is formed.
Endgroup-functionalized polymers have already been known for a long time. In U.S. Pat. No. 4,758,626, for example, polyesters were impact modified using carboxy-terminated polyacrylates. However, no description was given there of specific endgroup crosslinking.
U.S. Pat. No. 4,699,950 describes thiol-functionalized polymers and block copolymers. The polymers, however, contain only one functional group, which is subsequently used for polymerization or for other reaction.
U.S. Pat. No. 5,334,456 describes maleate- or fumarate-functionalized polyesters. Subsequent crosslinking takes place in the presence of vinyl ethers. Here again, polyacrylates are not described.
U.S. Pat. No. 5,888,644 describes a process for preparing release coating materials. Its starting point is formed by polyfunctional acrylates, which are reacted with polysiloxanes. Here again, no defined network is formed, so that this process cannot be transferred either to acrylic PSAs.
U.S. Pat. No. 6,111,022 describes poly(meth)acrylonitrile polymers prepared by ATRP. Terminally functionalized polymers can also be prepared by these processes. Advantageous processes for preparing purposively crosslinked PSAs are not disclosed, however.
In U.S. Pat. No. 6,143,848, terminally functionalized polymers are prepared by a new, controlled polymerization process. The polymerization process employed is an iodine transfer process. However, polymers of this type lack great thermal stability, since iodides generally react with air and are easily oxidized to iodine. Severe discolorations are a consequence of this. This applies in particular to hotmelt processes with high temperatures.
None of the aforementioned documents points to a process in which polyacrylates endgroup-functionalized with photoinitiators have been deliberately reacted therewith in order to construct a linear polymer chain or a polymer network.
It is an object of the invention to specify a process for building up the molecular weight of polyacrylates, in particular for their crosslinking, which has the disadvantages of the prior art only to a reduced extent, if at all.
Surprisingly, and unforeseeably for the skilled worker, this object is achieved by the process of the invention, as specified in the independent claim and in the subclaims.